1. Field of the Invention
The invention relates to optical reflectors, in particular for construction or geodetic surveying, according to the preamble of claim 1.
2. Description of the Related Art
Such reflectors have been used for decades in surveying as target objects for distance-measuring apparatuses or position-determining apparatuses with automatic target acquisition. The electromagnetic waves emitted by these apparatuses, for example laser beams in infrared or visible wavelength ranges are reflected back to the respective apparatus by the reflectors. For trouble-free and reliable measurement, it is necessary for many applications that such a reflector reflects the beams in a specific direction and with high intensity.
Directed reflections having such a required intensity can be achieved, for example, by means of reflective bodies which have three plane reflection surfaces oriented perpendicular to one another and a plane transmission surface for the beams which is tilted relative to each reflection surface. Reflective bodies formed in this manner are called triple prisms, reflector prisms, cube-corner reflectors or retro-reflectors.
The reflection of the beams at the reflection surfaces should take place by total reflection at interfaces between the reflective bodies and the gas surrounding the reflection surfaces. If, on the other hand, interfaces are wet with liquids or droplets of condensed water vapor are deposited on them, as a rule total reflection no longer occurs at the interfaces or only in parts or with reduced intensity. This leads, inter alia, to a reduction in the distance over which distance-measuring apparatuses are used or in the achievable positional accuracy of position-determining apparatuses.
Since reflectors for surveying purposes are set up in the open air, in some cases also over relatively long periods, they are exposed to weather influences, such as, for example, rain, mist and sunlight. In order reliably to prevent wetting or misting of the respective reflection surfaces, the reflection surfaces of conventional reflectors are either surrounded by an inert gas atmosphere, for example of pure nitrogen, or provided with a reflection-promoting coating.
On the one hand, such a reflection-promoting coating of a reflection surface, for example a copper, aluminum, silver or gold layer applied by vapor deposition in a vacuum, can increase the production costs of such a reflective body by a half. On the other hand, a gas-tight housing which prevents the diffusion of water vapor is required for permanently maintaining the inert gas atmosphere around a reflection surface.
In addition, the inert gas must not be lost even in the case of a pressure increase inside the receptacle housing, which inevitably occurs during any warming up by sunlight or during transport in a transport aircraft. However, the provision of a gas-tight and pressure-resistant receptacle housing in turn requires a considerable manufacturing effort. The use of economical plastics materials is not suitable in practice for the production of gas-tight receptacle housings since these generally do not completely prevent the diffusion of water vapor.
It is therefore the object of the invention to eliminate deficiencies of the prior art. Moreover, it is intended to propose an economical reflector which reliably reflects beams in a directed manner and can be set up for a relatively long time in the open air.
This object is achieved by a reflector in which the defining features of the independent claim 1 are realized.
When a reflector for beams is mentioned in the context of the invention, it is intended to be understood as meaning reflectors having a reflective body of transparent material, for example of glass or plastic, which has at least one reflection surface and one transmission surface for the beams. These surfaces may be plane, spherical, aspherical or even in principle arbitrary, for example in the form of a freely shaped surface, depending on the application.
In the case of a reflector according to the invention, the reflection takes place asxe2x80x94is knownxe2x80x94at least one reflection surface by total reflection. However, a complicated and expensive reflection-promoting coating of the reflection surfaces is not necessary. The reflective body is held in a receptacle housing which protects the at least one reflection surface from weather influences which might otherwise impair the reflective power of the reflection surface. For this purpose, the cavity bounded by the reflection surfaces and the receptacle housing is sealed substantially water-fight from the environment of the reflector. According to the invention, this receptacle housing has a means for exchanging gas which is known per se and permits automatic exchange of gas in the cavity with gas in the environment. Such an automatic exchange of gas takes place, for example, owing to different absolute or partial gas pressures inside and outside the cavity, which are caused, inter alia, by changes in the air pressure, in the atmospheric humidity or in the outside or inside temperature.
Such a means for exchanging gas may be, for example, in the form of a semipermeable membrane which on the one hand is impermeable to water and, on the other hand, permeable to gas. Such semipermeable membranes known per se are already used in electronic apparatuses which are also designed for use in the open air and whose electronic components must be reliably protected from water. For example, microporous disks of rolled PTFE are used as means for exchanging gas.
The provision, according to the invention, of a means for exchanging gas results in an automatic, controlled gas exchange and, associated therewith, also a compensation of the relative humidity between the cavity of the reflector and its environment. After a time span which is negligible in practical use, the atmospheric humidity in the cavity then corresponds to the atmospheric humidity of the environment.
By mounting a means for exchanging gas, the formation of a deposit on the reflection surfaces can be prevented even in the case of a rapid drop in the outside temperature. Reflectors according to the invention can be reliably sighted with electro-optical distance-measuring apparatuses or position-determining apparatuses with automatic target acquisition.
As a result of a means for exchanging gas being mounted according to the invention, the requirements with respect to the material and the design of a receptacle housing for an uncoated reflector are much less restricted. Thus, for example, water vapor diffusion can be permitted to a certain extent. The use of economical plastics which can be efficiently processed is possible for the production of such receptacle housings.
Moreover, the means for exchanging gas ensures that virtually the same gas pressure prevails inside and outside a reflector according to the invention. Even a drastic pressure drop, as may occur, for example, after take off of a transport aircraft in the loading bay of the latter, cannot cause a reflector according to the invention to burst since the excess pressure can automatically escape via the means for exchanging gas. Accordingly, the receptacle housing can be made thin-walled.
According to the invention, it is furthermore proposed to provide a reflector with a fastening means which, in contrast to the prior art, has an internal thread instead of an external thread. On the one hand, an internal thread is better protected from damage from mechanical impact than an external thread and, on the other hand, the internal thread coordinated with the reflector provides a flexible fastening interface for a large number of reflector applications, for example in mining, tunnel construction and earthworks or in the monitoring of objects, since fastening to standardized threaded bolts, threaded pins, through-anchors provided with corresponding threads or other xe2x80x9cdirect fixing systemsxe2x80x9d is easily possible.
A reliable reflection of the beams by the reflective body with high intensity primarily requires reliable reflection of the beams at the at least one reflection surface. However, transmission of the beams in as undisturbed a manner as possible through the transmission surface of the reflective body is also necessary. By partial wetting of the transmission surface with raindrops or snowflakes, the beams may additionally be refracted in a manner dependent on position. Consequently, the intensity of the reflection is reduced. Particularly in the case of reflectors set up stationary in exposed areas, wetting of the transmission surface can occur. Such wetting can be prevented by a component which is detachably mounted according to the invention on a corresponding reflector.
In the case of the formation of the component, a compromise must be made between the protection desired and the dimensions of the component. Components dimensioned too large are not only bulky but offer large surfaces for attack which can lead to undesired misadjustment of the reflectors in strong wind. Of course, detachable mounting of such components can give rise to the advantageous effect described above not only on reflectors according to the invention.
Further advantageous or alternative designs or developments of the invention are described in the features of the dependent patent claims.